This invention relates to a radar device to be carried on an automobile for measuring on real time the distance to a front-running vehicle.
The radar device carried on an automobile is a kind of so-called pulsed radar device for transmitting a pulsed beam of electromagnetic waves forward and measuring the distance to an object in front inclusive of a vehicle which may be accelerating, decelerating or even stationary (hereinafter referred to as the front-running vehicle) or its relative speed on the basis of the time it takes to receive its reflection. It now goes without saying that ordinary visible light and x-rays are examples of electromagnetic waves.
FIG. 10A is a conceptual diagram showing the principle of a prior art radar device 1 carried on an automobile. A pulsed beam 3 of electromagnetic waves transmitted from its signal transmitter (TX) 2 is reflected by a body surface 4 (or any reflective surface such as a back reflector) of a front-running vehicle and received by its signal receiver (RX) 5. If the time between the transmission of the beam and the reception of the reflected beam is T as shown in FIG. 10B, the distance L to the front-running vehicle is given by cT/2 where c is the speed of light. The relative speed between the front-running vehicle and one""s own vehicle carrying the radar device 1 can be calculated from the time-rate of change in the distance L between the two vehicles, or as the slope of the curve on the graph of L plotted against the time. If the change in L along the time-axis is zero, for example, this means that the relative speed is zero, or that the front-running vehicle is running at the same speed as one""s own vehicle. If L is increasing with time, this means that the front-running vehicle is accelerating with respect to one""s own vehicle, and if L is decreasing with time, this means that the front-running vehicle is decelerating with respect to one""s own vehicle.
Since the radiative energy of transmitted electromagnetic waves generally decreases inversely proportional to the fourth power of distance, it must be sufficiently large in order to obtain a sufficiently intense reflected beam. Since the capability of the signal transmitter 2 is limited, the beam 3 is generally patterned in a narrowed form, say, with the angle xcex8 of the range of vision equal to about 4xc2x0. Such a narrowed pattern is preferred also for the purpose of improving the directional resolution. FIG. 11A shows an example of narrowed beam pattern in the horizontal direction. FIG. 11B is an example of narrowed beam pattern in the vertical direction. FIG. 11C is an example of the cross-sectional shape of such a narrowed beam. Although an example of a narrowed beam with a nearly circular cross-sectional shape is illustrated, the angle xcex8 of the range of vision need not be equal in the horizontal and vertical directions. Such a narrowed beam is generally transmitted at a specified elevation angle (the elevation angle shown in FIG. 11B being zero) while scanning in the horizontal direction within a specified range, as shown in FIG. 11A. The range of the scanning may be determined such that the entire width of an automobile at a sufficiently large distance from one""s own vehicle can be covered, that is, about 2.5 m-3 m at 10 m.
With a prior art radar device 1 thus structured, a beam 6 is transmitted from one""s own vehicle 7 as shown in FIG. 12A and if its reflection from the front-running vehicle 8 indicates that the distance between the two vehicles 7 and 8 is sufficiently large, one""s own vehicle 7 may be accelerated to reduce the distance. If the distance in between is found to be too small to be safe, such as shown in FIG. 12B, one""s own vehicle 7 may be braked so as to avoid a collision. In this manner, a so-called stop-and-go system for creeping forward in a traffic jam while maintaining a constant distance from the front-running vehicle may be realized.
With a prior art radar device 1 as described above, a front-running vehicle at a certain distance can be reliably kept visible because a narrowed beam 6 is made used of. There is a problem of suddenly losing sight of the front-running vehicle, however, when the distance between the vehicles is very short. FIGS. 13A and 13B show an example of such a situation where one""s own vehicle 7 may be a sports car and is relatively low while the front-running vehicle may be a large freight truck having a back reflector attached at a relatively high position. Since the radar beam 6 is usually transmitted with an elevation angle of about zero degree and a range of vision of about 4xc2x0, the front-running vehicle 8 is safely visible as long as it is at a sufficiently large distance, as shown in FIG. 13A. When the distance between the two vehicles 7 and 8 is small as shown in FIG. 13B, however, the beam 6 goes under the body of the front-running vehicle 8 without reaching its reflector at the back.
FIGS. 14A and 14B show another example of such a situation where one""s own vehicle 7 may be a large freight truck while the front-running car 8 may be a sports car and is relatively low. When the distance between the two vehicles 7 and 8 is sufficiently large, the front-running vehicle 8 remains within the spreading range of vision of the beam 6, as shown in FIG. 14A. When the distance between the two vehicles 7 and 8 is small as shown in FIG. 14B, however, the beam 6 may pass above the highest reflective part of the front-running vehicle 8, thereby inconveniently losing sight of it.
These two examples show that necessary data can be reliably obtained as long as the front-running vehicle 8 is sufficiently far away because its presence can be monitored by the radar device 1 on one""s own vehicle 7 but the front-running vehicle 8 may be xe2x80x9clostxe2x80x9d if the distance between the vehicles 7 and 8 suddenly decreases, say, because the front-running vehicle 8 has suddenly decelerated. From the point of view of safety requirement on such a device, the aforementioned problem is one that must be solved.
It is therefore an object of this invention to provide an improved radar device to be carried on an automobile with which the problem of suddenly losing sight of the front-running vehicle can be solved.
A radar device embodying this invention, with which the above and other objects can be accomplished, may be characterized not only as comprising a transmitter for transmitting forward a beam of electromagnetic waves having a specified vertical angular range of vision at a specified elevation angle (the xe2x80x9cspecified initial elevation anglexe2x80x9d), a receiver for receiving reflected waves of the transmitted beam from a vehicle traveling in front, and a measuring device for measuring a distance to the vehicle in front based on outputs from the receiver, but also as including a command outputting means for outputting a command signal when the distance measured by the measuring device is decreasing and reaches a certain threshold distance below which the measuring device becomes incapable of measuring the distance based on the outputs from the receiver and a beam adjusting means for changing either the elevation angle or the angular range of vision of the transmitted beam in response to this command signal.
In the above, the threshold distance is the distance at which the reflecting portions such as reflectors at the back of the vehicle in front come to be at a dead angle, not reachable by the beam which is directional, usually having a very small angular range of vision. This can happen most frequently where the difference in height between the transmitter of the beam and the reflector on the vehicle in front is great, and this threshold distance can be calculated from this height difference and the angular range of vision of the transmitted beam.
When the distance to the vehicle in front measured by the measuring device is decreasing and reaches this threshold distance, the radar device concludes that there is a high possibility that the vehicle in front which has been sending back the reflected beam is still in front although the reflected beam may cease to be received and outputs a specified command signal. The means for outputting this command signal is hereinafter referred to as the xe2x80x9ccommand outputting means.xe2x80x9d In response to this outputted command signal, the radar device causes a change in the emitted beam of radiation either by changing its elevation angle or its angular range of vision such that the reflected waves from the vehicle in front will continue to be received by the receiver and the measuring device will continue to calculate the distance to the vehicle in front. The mechanism for thus modifying the emitted beam is hereinafter referred to as the xe2x80x9cbeam adjusting means.xe2x80x9d
If the beam adjusting means is for changing the angular range of vision of the beam to be emitted, the angular range of vision is changed so as to be increased such that the front-running vehicles can continue to reflect back the emitted waves. If the beam adjusting means is of the type for changing the elevation angle of the emitted beam, the elevation angle may be changed either upward or downward. The radar device includes a height setting unit for changing (say, manually) the height of the point of emission of the laser beam. If the emission point is at a higher of the settable positions, the beam adjusting means will function to change the elevation angle downward. If the emission point is at a lower of the settable positions, the beam adjusting means will function to change the elevation angle upward. In this manner, the beam direction can be shifted and the laser beam can go after the front-running vehicle which may have escaped into one of the blind angle regions.
It is preferable to also provide a means (hereinafter referred to as the xe2x80x9creturn signal outputting meansxe2x80x9d) for outputting a signal (hereinafter referred to as the xe2x80x9creturn signalxe2x80x9d) after the command signal is outputted when the distance measured by the measuring means becomes greater than the aforementioned threshold distance (or when the outputs from the receiver increases and change from lower to higher than a threshold level below which the measurement device becomes incapable of measuring the distance therefrom to the front-running vehicle), and a returning means for causing the shifted elevation angle to return back to the specified initial angle or the vertical angular range of vision to return back to the specified angular range in response to the return signal. Thus, the elevation angle or the angular range of vision of the emitted beam of waves is returned to the original state when the distance to the front-running vehicle is restored to a safe range greater than the threshold distance, or when the receiver begins to receive the reflected waves from the front-running vehicle.
It is preferable, furthermore, that the aforementioned command outputting means will function to check whether the front-running vehicle is traveling in the same traffic lane as the automobile on which the radar device is installed and output the command signal only if it is ascertained that they are in the same lane. With such a command outputting means, the radar device can be useful even while the automobile is traveling on a multi-lane highway.